As a process for forming automotive upholstery members, the mold press forming process has been widely employed. According to the mold press forming process, resin in semi-molten state is distributed to the upper and lower dies for mold press forming, and the upper and lower dies are closed upon each other, so that the shapes of the molded members can be freely selected, and the material loss can be minimized.
In this mold press forming process, as illustrated in FIG. 7, resin material is molded between upper and lower dies 1 and 2 for mold press forming which can be closed upon each other with a prescribed pressure and are provided with prescribed die surfaces, and the material is supplied to the dies from an injection molding machine 3 connected to the lower die 1.
More specifically, as illustrated in FIG. 8, molten resin is supplied to a hot runner 5 provided in a manifold passed inside the lower die 1 from a nozzle 4 of the injection molding machine 3, and is distributed to the die surface of the lower die 1 via a plurality of valve gates 6 branching off from the hot runner 5. The so-called multi-point gate method is most widely used in which a plurality of such valve gates 6 are distributed over the die surface.
According to the mold press forming process based on the multi-point valve gate method, it is necessary to control the amount of resin that is supplied to each of the valve gates 6a, 6b and 6c according to the shape and thickness of the product, and various methods have been practiced for this purpose. According to the temperature control method, the amount of resin is controlled by changing the set temperature of each of the valve gates 6. According to another method, the amount of resin supplied to each of the valve gates is controlled by using a timer.
Normally, the opening and closing timing of each of the valve gates 6a, 6b and 6c is controlled according to a timing chart such as the one illustrated in FIG. 9 for the purpose of supplying a prescribed amount of resin to each of a plurality of locations in the lower die 10.
Thus, according to the conventional method for mold press forming based on the use of valve gates, for instance, if a timer control is employed, the opening period of the valve gates can be controlled by the order of 0.1 seconds, but due to the change in the flow resistance in the hot runner 5 the amount of resin supplied from each of the valve gates cannot be accurately controlled, and it is known that some unevenness in the amounts of resin supplied to different locations of the die surface often occurs.
This is caused by the fact that the fluidicity of the molten resin tends to differ from one gate to another due to the unevenness in the temperature distribution in the hot runner, and that the presence of residual unmolten resin in the gates, in particular at the time of start-up, and the introduction of foreign matters into the molten resin tend to cause unevenness in the fluidicity of the molten resin, and may even cause the blockage of part of the gates.
Therefore, the resulting difficulty in controlling the thickness of the product sometimes caused warping of the product, excessive generation of burrs, and creation of voids.
Further, the thermal capacity of the product in relation with a surface skin member placed on the product could not be properly controlled, and unevenness in the thermal capacity tends to impair the external appearance of such a surface skin member when it is placed over the surface of the molded product to form an integral assembly.